Insulated cable with sheath of controlled peel strength and method

ABSTRACT

A high voltage electrical cable having at least two layers of cross-linked polyolefin insulation, i.e. polyethylene; the external layer being semi-conductive and strippable from the internal layer. Controlled strippability is accomplished by sulfonating the respective layers at their contact interface.

United States Patent [191 Carini et al.

INSULATED CABLE WITH SHEATH OF CONTROLLED PEEL STRENGTH AND METHODInventors: Francis F. Carini, Pittsburgh, Pa.; Steven R. Abbott,Vincennes, 1nd.

Assignee: Essex International, Fort Wayne,

Filed: May 30, 1972 Appl. No.: 258,077

US. Cl 156/51, 117/47 N, 117/62.1,

117/62.2, 117/232, 156/307, 156/244, 156/247, 174/110 PM, v174/120 R,174/120 SR, 174/120 SC, 260/686 Int. Cl. H01b 13/14 Field of Search.....156/51, 272,244, 306, 308; 260/793, 686; 117/47 R, 47 A, 62-622,117,118, 216, 218, 224, 232; 174/120 SC, 120 SR, 120 R, 110 PM, 102 SC,106 SC; 161/188, 411

[ 1 Jan. 22, 1974 [56] References Cited UNTTED STATES PATENTS 3,705,25712/1972 Wade 174/120 SC X 3,541,228 11/1970 Lombardi 174/102 SC X3,646,248 2/1972 Ling et a1 174/120 SC 3,479,446 11/1969 Arnavdin et a1.156/51 X 3,684,821 8/1972 Miyauchi et a1. 117/232 X 3,624,054 11/1971Barton et a1. 260/79.3R 3,586,569 6/1971 Caiola 156/308 Primary ExaminerAlfred L. Leavitt Assistant ExaminerDavid A. Simmons Attorney, Agent, orFirm-A. W. Molinare et al.

ABSTRACT A high voltage electrical cable having at least two layers ofcross-linked polyolefin insulation, i.e. polyethylene; the externallayer being semi-conductive and strippable from the internal layer.Controlled strippability is accomplished by sulfonating the respectivelayers at their contact interface.

5 Claims, 1 Drawing Figure 1 INSULATED CABLE WITH SHEATH OF CONTROLLEDPEEL STRENGTH AND METHOD BACKGROUND OF THE INVENTION This inventionrelates to an improved high voltage cable having an insulating layer ofvulcanized polyolefin and an outer semi-conductive layer of toughvulcanized polyolefin wherein the layers are strippably bonded.

In particular, this invention relates to an improved high voltage cablewhich has an inner semi-conductive shield of vulcanized polyolefin, aninsulating layer of vulcanized polyolefin and an outer semi-conductorshield of tough vulcanized polyolefin which is readily strippable fromsaid insulating layer to facilitate splicing said cable, yet issufficiently bonded so that the semi-conductor layer does not separateduring installation and conventional use. The outer extrudedsemiconductor layer can be-tandem extruded and simultaneously vulcanizedwith insulating layer.

It has been proposed (U.S. Pat. No. 3,541,228) to make a high voltagecable having strippable layers by using a chlorosulfonated polyethylenesemi-conductor compound for the outer sheath. Such compounds, whilestrippable, do not have the desired toughness and heat resistance. Theselatter toughness and heat resistance properties are obtainable in across-linked or vulcanized semi-conductive polyolefin such aspolyethylene or copolymers thereof but during vulcanization, thepolyolefin layer cross links with the insulating layer beneath it andbonds so tenaciously that it is difficult to peel for splicing. I

Attempts have been made to reduce adhesion between the two insulatinglayers. For example, a silicone coating has been placed on the outersurface of the inner insulating layer prior to laminating with thesemiconductive compound and vulcanizing. However, the presence of thesilicone rendered the polyethylene surface too slippery to be practical.The outer cover stripped readily but separations from the inner layeroccurred under normal conditions of use.

It is well known that the laminated layers on the conducting metalmember of the cable must be bonded together with few, if any, air orvoids between layers. Consequently, any adhesive bond produced must bestrong enough to prevent the formation of voids or air pockets duringfabrication of the cable or upon use in service. For example, accordingto Specifications for Polyethylene and Cross-linked PolyethyleneInsulated Shielded Power Cables Rated 2,001 to 35,000 Volts, publishedby the Association of Edison Illuminating Companies (AEIC No. 5-69), theteachings of which are specifically incorporated by reference herein,for thermoplastic polyethylene insulation there shall be no voids whoselargest dimension exceeds 2.0 mils. For cross-linked thermosettingpolyethylene, there shall be no voids whose largest dimension exceeds5.0 mils. In addition, the maximum number voids shall not exceed aspecified number of voids per cubic inch of insulation.

It has also been proposed to reduce the bond strength at the interfacebetween the two layers by oxidizing the outer surface of the innerpolyethylene layer. The oxidation was carried out by subjecting thesurface to a corona discharge, a gas flame or to ozone gas to decomposethe cross-linking agent in the polyethylene compound and to tie upreactive groups. The problem in producing a practical laminate byoxidation involved control. Excessive oxidation degraded the surface tosuch an extent that no bonding occurred. On the other hand, insufficientoxidation did not decompose the cross-linking agent, and the bond, aftervulcanization, was too tenacious. To. produce the desired degree ofoxidation between these limits proved to be difficult.

SUMMARY OF THE INVENTION An object of this invention is to provide aninsulated electrical cable of the kind described having an innervulcanized polyolefin insulation layer, and an outer semi-conductivevulcanized polyolefin layer, the bond between said layers permittingstripping of the outer layer for splicing, but being sufficientlytenacious to preclude separation except when stripping.

It is a further object of this invention to provide a method forstrippingly bonding an outer vulcanizable semi-conductive polyolefinlayer to an inner vulcanizable polyolefin insulating layer in a readymanner.

In preparing an insulated cable for splicing, it is important that, whenthe semi-conducting layer is stripped, no conductive material is left onthe surface of the inner insulating layer. For example, if conductivecarbon is left on the insulation, a ground potential may be createdwhich will result in failure of the insulation at the splice. It isalso' important that the semiconducting outer layer adhere to theinsulating layer so that the voids or air spaces in the laminate whichcan result in electrical losses, are kept to a minimum as previouslyindicated.

In accordance with our invention, these objects are accomplished bysulfonating the surface of the vulcanized polyolefin insulating layerprior to enclosing it in the semi-conducting outer layer. By attachingsulfonate groups to the polyolefin molecule, diffusion of thecross-linking agent across the interface of the laminate is arresed orsubstantially retarded. This prevents the two layers locking togetherduring vulcanization. It is suspected that because the sulfonate groupswhich form at the surface are relatively bulky they retard movement ofthe cross-linking agent across the interface. In a preferred form of theinvention, sulfur trioxide gas in an inert carrier gas, such as,nitrogen, is passed over the surface of the unvulcanized polyolefin. Theconcentration of the sulfur trioxide, the temperature and time ofexposure must be controlled to produce a strippable bond of desiredstrength without degrading the polyolefin surface by excessivesulfonation.

The desired strength of the bond can be controlled to amounts less that16 pounds and preferably in the range of about 12-12 pounds measured, bypeeling at an angle of to the interface, a inch width flat strip of thelaminated semi-conductive polyolefin material. The exact method fordetermining stripping strength is described by AEIC No. 5-69specification I-I.l.l. According to this specification, thesemi-conducting jacket of a cable sample approximately 15 inches long iscut longitudinally and vertically down to the insulation. A second cutis then made in the same way at a 1% inch separation parallel to thefirst cut. Approximately 2 inches of the V2 inch strip of each end ofthe cable is removed by pulling it at a 90 angle away from the cable. Ahole is punched at each end of the strip and the sample mountedhorizontally on a scale hook through the holes. The stripping strengthor tension is then determined by increasing the force on the strip at a90 angle to the cable until the strip separates from the insulation at aspeed of approximately one-half inch per second. As indicated, therequired tension is preferably not less than 2 pounds nor more than 12pounds although stripping strengths of -16 pounds are within the scopeof the present invention.

Vulcanizable polyolefins suitable for use in accordance with the presentinvention include homopolymers and copolymers of the low molecularweight alpha olefins such as ethylene and propylene. Preferred areethylene homopolymers, ethylene-propylene copolymers, chlorinatedpolyethylene and ethylene-vinyl acetate copolymers as commonly used forcoating or jacketing electric cables. Preferred are thermoplasticpolyethylene, commonly referred to in the art as polyethylene andcross-linked thermosetting polyethylene, commonly referred to in the artas cross-linked polyethylene.

In preparing insulated cable in accordance with the invention, ametallic conductor such as a bundle of aluminum or copper wirespreferably having an inner semi-conducting Vulcanizable polyolefin orcopolymer layer and subsequent extruded cover of Vulcanizable polyolefinis passed continuously through a cylindrical chamber containing about0.10 to 2 percent by volume sulfur trioxide in nitrogen. The length ofthe chamber and the rate at which the cable passes through the chamberis controlled so that the polyolefin insulating jacket is exposed for aperiod of between 0.1 and 30 seconds, preferably between.0.l andseconds. Particularly suitable reaction temperatures are within therange of about 16C. to about 200C. Satisfactory peelable bonds forpolyethylene have been achieved after A2 second exposure to 1.5 percentsulfur trioxide-in nitrogen at room temperature. At a concentration of0.5 percent sulfur trioxide, the exposure time must be in-. creased to2.5 or 3.0 seconds. At concentrations of 2 percent sulfur trioxide,exposure must be limited to less that 25 seconds because charring of thepolyethylene will begin after this amount of exposure. The reactant gasis prepared by metering quantities of sulfur trioxide from a pot heatedabove 44C. into a metered quantity of nitrogen. The percentages of thegases are by volume. The gas chamber is equipped with an entrance portnear one end and an exit port near the other. The sulfur trioxide flowsthrough the chamber at a rate to maintain substantially constantconcentration of sulfur trioxide within the chamber. A seal is providedat the margins of entrance and exit openings of the chamber to preventescape of gas around'the jacketed conductor being treated.

The insulated conductor emerging from the reactor is then fed into anextruder where a semi-conductive layer of a polyolefin such aspolyethylene is applied over the sulfur trioxide-treated surface. Thelaminated cable is then subjected to vulcanization such as a temperatureof 170C for times of about -40 minutes to cure the polyethylene andestablish the peelable bond at the interface.

A preferred Vulcanizable polyethylene for the insulating layer iscompounded with dicumyl peroxide curing agent and sold as Polycure 521by Cooke Color and Chemical Company, Hackettstown, NJ. When vulcanized,this material has a tensile strength of about 2,500 psi and anelongation of 550 percent. Its heat distortion at 121C is 14 percent andthe composition is characpropylene, vinyl monomers, i.e. vinyl esters,vinyl ha lides such as vinyl chloride, or vinyl acetate andacrylonitrile. The copolymers preferably contain from -95 ethylene andthe remainder being alpha olefinic material. To make such materialssemi-conductive, about 30 to 40 parts of conducting carbon black aremilled into the polymer or copolymer resin. A preferred semiconductiveVulcanizable copolymer is sold by Union Carbide Corporation of New YorkCity under the designation HFDA-OS Black 55.

The exact peel strength to be obtained for a given jacket or layer isdependent on the particular characteristics of the material such as thebulk mechanical properties; the physical and chemical nature of thesurface and the nature of the interface that is formed. Each of thesevariables must be specified as closely as possible. For example, as thechemistry or roughness of a surface changes, different amounts ofsulfonation are required to obtain a. particular peel strength.Thus,'for two separate cables, with no sulfonation, if the peel strengthis 8 pounds, and 25 pounds respectively, then less sulfonation would beneeded to lower the peel strength to 4 pounds for the 8 pound cable thanfor the 25 pound cable.

The exact degree of sulfonation or surface polarization of thepolyethylene may be measured by determining the infrared absorption ofthe sulfonated polyethylene surface, the absorbence ratio beingproportional to the degree of sulfonation. For polyethylene surfaces,this degree of sulfonation is calculated as the ratio of the absorptioncoefficient of the peak 1160 cm divided by the adsorption at z 1,460 cm.The 1,160 peak is mentioned in Polyethylene Surfaces III by Olsen andOsteraas as due to vibrations of the sulfonic acid. The 1,460 peak isdue to CH and CH deformation. In general, the larger the ratio, the moresulfonation has occurred. The absorbtion due to the CH and CH should bea normalizing factor. The preferred absorbence ratio is from 0.5 to 0.75which results in a bond strength ranging from 6 to 10 pounds per V2 inchwidth strip. At an absorbence ratio of 0.25 to 0.35 the polyethylenelayers bond so tenaciously after vulcanization that one cannot be peeledfrom the other. At an absorbence ratio of 2.0, the layers for certainapplications may strip too easily and may result in undesirable airpockets and separations.

For the described Polycure 521 Vulcanizable polyethylene, at 27C acontact time of about 4 seconds with a 1 percent 50;, in nitrogenmixture gave a sulfonation ratio of between 0.5 and 1.0.

Peel strength may also be correlated with the wetting angle of a drop ofwater on the sulfonated surface. The

' surface having a wetting angle of from 5070 provides the desirable 2to 12 pounds peel strength.

The cable of the invention is shown in the attached drawing.

Copper conductor 1 is enclosed in an internal layer of vulcanizablesemi-conductive copolymer ethylene 2, and in an external layer ofvulcanized polyethylene 3 which, in turn, is bonded by means of theprocedure described herein to an external layer of vulcanizablesemiconductive copolymer ethylene 4. The layer 4 may be readily strippedfrom the insulation 3 without leaving any conductive carbon or otherresidue. Thus, the cable of the invention is readily suited for splicingor terminating.

It is to be realized that the cable illustrated merely illustrates apreferred three-layer cable configuration and cables containing twolayers as well as cables containing more than three layers are withinthe scope of the present invention.

In place of gaseous sulfur trioxide, the sulfonation of the inventionmay be carried out .by exposing the polyethylene surface to fumingsulfuric acid or sulfonyl chloride.

What is claimed is:

l. A method for producing an adhesive bond of predetermined strengthbetween the first insulating layer of a vulcanizable polyolefin of ahigh voltage electric cable and a second external layer ofsemi-conductive 6 polyolefin polymer which comprises the steps of:

i. extruding the vulcanizable polyolefin around the cable conductor assaid first layer;

ii. exposing the unvulcanized surface of said polyolefin to asulfonating agent at sulfonating conditions for a time sufficient tosulfonate a portion of the first, insulating layer, thereby renderingsaid surface incapable of complete reaction with said second, externallayer;

iii. extruding said second external sheath over said sulfonated surface;and

iv. subjecting the cable to elevated temperature to vulcanize saidsecond layer and cross-link the still reactive portion of saidsulfonated surface of the first layer to said second layer to producesaid adhesive bond of predetermined strength.

2. A method according to claim 1 wherein said cable conductor of step(i) has a layer of cross-linked, semiconductive polyolefin extrudedthereon prior to the extrusion of the first insulating layer.

3. A method according to claim 1 wherein said first insulating layer isa peroxide vulcanizable polyolefin.

4. A method according to claim 3 wherein said polyolefin in said firstand second layer is polyethylene.

5. The method of claim 1 in which said sulfonating agent is a mixture of0.5 to 3 percent by volume of SO in an inert carrier gas.

2. A method according to claim 1 wherein said cable conductor of step(i) has a layer of cross-linked, semi-conductive polyolefin extrudedthereon prior to the extrusion of the first insulating layer.
 3. Amethod according to claim 1 wherein said first insulating layer is aperoxide vulcanizable polyolefin.
 4. A method according to claim 3wherein said polyolefin in said first and second layer is polyethylene.5. The method of claim 1 in which said sulfonating agent is a mixture of0.5 to 3 percent by volume of SO3 in an inert carrier gas.